Sunday, February 14, 2010

"If It’s That Warm, How Come It’s So Darned Cold?"

by James Hansen, Reto Ruedy, Makiko Sato, Ken Lo

An Essay on Regional Cold Anomalies within Near-Record Global Temperature

Overview. Public skepticism about global warming was reinforced by the extreme cold
of December 2009 in the contiguous 48 United States and in much of Eurasia. The summer of
2009 was also unusually cool in the United States. But when a cold spell hits, we need to ask:

* Cold compared to what. Our memory of the past few winters? Winters of our
childhood? Winters earlier in the 20th century?
* Cold where and for how long? Regional cold snaps are expected even with large
global warming. Weather fluctuations can be 10, 20 or 30 degrees, much larger than average
global warming.
* The reality of seasons. As the plot of Earth we live on turns away from the sun, in
winter or at night, it cools off. That’s true even with global warming, albeit not quite so much.

Our data show that 2009 was tied for the second warmest year in the 130 years of near-global
instrumental measurements – and the Southern Hemisphere had its warmest year in that entire period.
Before discussing these data, and their reconciliation with regional cold anomalies, we must consider
the time frame of comparison.

If we look back a century, we find cold anomalies that dwarf current ones. Figure 1 shows
photos of people walking on Niagara Falls in 1911. Such an extreme cold snap is unimaginable today.
About a decade earlier, in February 1899, temperature fell to -2°F in Tallahassee, Florida, -9°F in Atlanta,
Georgia -30°F in Erasmus, Tennessee, -47°F in Camp Clark, Nebraska, and -61°F in Fort Logan, Montana.

The Mississippi River froze all the way to New Orleans, discharging ice into the Gulf of Mexico.

As we will show, climate is changing, especially during the past 30 years. The changes are
perceptible, even though average temperature change is smaller than weather fluctuations. The answer
to the simple question: “How come it’s so damned cold” turns out to be simple: “Because it’s winter.”...

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